1. Field of the Invention
The present invention relates to a process for preparing organophilic clay gellants in general and a process for preparing an organophilic clay gellant which is particularly useful for thickening polyester compositions. The present invention also relates to organophilic clay gellants which may be prepared by the processes and to a process for thickening a polyester composition.
2. Background of the Invention
It is well known in the art that organophilic clays can be used to thicken a variety of organic compositions. A problem which is often encountered in using organophilic clay gellants is that to obtain the desired level of thickening, it is necessary to disperse the gellant thoroughly in the composition. Otherwise, a much greater amount of organophilic clay gellant is needed and/or the organophilic clay particles may be larger than other particles (e.g., pigment) present in the composition thereby leading to adverse results (e.g., an unattractive rough coating or settling of the particles).
In an effort to achieve proper dispersion of the organophilic clay gellant, the art has explored various avenues. For example, certain organophilic clay gellants, such as shown in U.S. Pat. No. 2,531,440, were mixed with the organic composition to be thickened and the mixture subjected to high shear conditions. Alternatively, low molecular weight polar organic materials known as polar activators, dispersants, dispersion aids, solvating agents, dispersion agents and the like, have been combined with the organophilic clay gellant in order to achieve dispersion into the organic composition. Illustrative patents which disclose these materials are U.S. Pat. Nos. 2,677,661, 2,704,276, 2,833,720, 2,879,229, 2,966,506 and 3,294,683.
More recently, organophilic clay gellants have been developed which are the reaction products of certain types of clays having a cation exchange capacity with certain organic cations or organic cations and organic anions. These gellants have the advantage that in some organic compositions they can be effectively dispersed without a dispersion aid under normal shear conditions. Illustrative patents which describe such improved organophilic clay gellants are U.S. Pat. Nos. 4,105,578, 4,208,218, 4,287,086, 4,391,637, 4,410,364, 4,412,018, 4,434,075, 4,434,076, 4,450,095 and 4,517,112.
One type of organic composition which has provided a particularly difficult environment in which to achieve effective and efficient dispersion of the organophilic clay gellant are polyester compositions. In the past, polyester compositions have been thickened using asbestos, fumed silica and certain types of organophilic clays. Each of these materials has certain drawbacks. Specifically, asbestos is known to present significant health hazards. Fumed silica forms a dust when handled, yields batch to batch variation in viscosity and will actually lose viscosity and performance if over-sheared. Additionally, fumed silica often reduces the clarity and shelf life of the polyester composition.
If an organophilic clay gellant is used to thicken polyester compositions, the dispersion of the gellant has been achieved by preparing a pregel under high shear conditions which is comprised of the gellant, an unsaturated monomer, such as styrene, and possibly a polar solvent. Such a technique is described in U.S. Pat. Nos. 3,974,125, 4,081,496 and 4,216,135. U.S. Pat. No. 4,240,951 also describes the use of organophilic clay gellants, such as prepared from dimethyl dihydrogenated tallow ammonium salts and dimethyl benzyl hydrogenated tallow ammonium salts, which can allegedly be added by direct addition, but is preferably added via the formation of a pregel. However, the patent further states that in this instance, numerous experiments are needed to determine what is referred to as the optimum level of the organic cation on the clay.